A comparison of two methods for estimating refill adherence to statins in Sweden: the RARE project

To analyse and compare refill adherence to statins estimated with two different methods with a focus on sensitivity to definitions.

Individuals aged 18-85 years who filled a statin prescription for the first time in 1.5 years during 1 January-30 June 2007 were followed until emigration or death or until 2 years after their first statin purchase. The data were collected via linkage between the Swedish Prescribed Drug Register, the National Patient Register and the Total Population Register. Days' supply was estimated based on amount dispensed and prescribed dosage. Refill adherence was estimated with the continuous measure of medication acquisition (CMA) and the maximum gap method (cut-off 45 days). The impact of altering definitions, for example, regarding hospitalisations, length of observation period and management of overlapping supply, was analysed.

The study included 36, 661 individuals (mean age 64 years, 47% women). The median proportion of days with statins was 95%, and 76% were classified as adherent with a cut-off at ≥ 80% with CMA. With the maximum gap method, 65% were adherent. Disregarding hospitalisations did not alter the results. Emigration or death at least one year after statin initiation was associated with a lower adherence with both methods, and a shorter observation period and adding overlapping supply to the subsequent prescription increased the adherence estimates.

The choice of method and definitions, particularly regarding the management of overlapping supplies and the length of observation period, has a substantial impact on estimates of refill adherence to statins.